The Government could face a legal challenge to controversial plans to introduce a threshold on union strike ballots.

Dave Prentis, general secretary of Unison, attacked proposed legislation making strikes unlawful unless 50% of those entitled to vote take part in a ballot, with a threshold of 40% support of everyone eligible to vote needed in public services.

Mr Prentis said the move was "vindictive" and warned the union could challenge it legally.

Speaking ahead of Unison's national conference in Glasgow, which opens tomorrow, he maintained the new law would make no difference to the union's strategy on industrial action.

He also warned Labour it could be out of power for a decade unless it reconnected with the public, developed more popular policies, and got "its act together".

He told the Press Association: "We have massive issues to face, irrespective of the Labour leadership. There have been huge cuts in the public sector, with hundreds of thousands of jobs lost, and more to come, which means that statutory local services will not be maintained.

"We are gearing up to resist the attacks on trade unions with every means at our disposal, and if necessary we will take legal action. The attacks are also an attack on civil liberties, so we will join with community groups and others to try to isolate the Tories.

"The future is in our own hands rather than thinking we can sit back and wait for Labour to win."

Mr Prentis revealed that 170,000 new members joined Unison last year, including 4,400 in the week of the general election in May.

Around 430,000 Unison members are eligible to vote in the forthcoming Labour leadership ballot, with a special conference being held next month before the union decides whether to recommend one of the four contenders.

Mr Prentis said it was possible the union would decide not to recommend anyone, a position which could be taken by other unions.

He has been angered by reports that unions were instrumental in electing Ed Miliband as leader, saying that at the time, one vote by an MP was worth 12,000 other party members, under an electoral college system which has been changed to one member, one vote.

"We have a new voting system, which I believe in, but we may well leave it to our members to decide. It is obvious that Labour failed to get its message across - no one knew what the party's position was or what alternative it was offering.

"Any Labour leader is going to have to address what the party stands for and what would make low-paid workers even register to vote. A fundamental reappraisal is needed, but not one based on the old Blairites who are now coming out of the woodwork, blaming trade unions for their failures."

Mr Prentis added that Unison would not be "cowed" by any new law on ballots, and would continue to organise industrial action if necessary.

Speaking ahead of today's 800th anniversary of Magna Carta, TUC general secretary Frances O'Grady said: "On the day when we rightly celebrate human rights and access to justice, our government is busy working on reforms aimed at shutting down dissent and weakening people's rights at work.

"The Conservative Trades Union Bill is a cynical attempt to suppress trade unions ahead of massive cuts to the public sector.

"Unions often are the last line of defence. The Government want to make it more difficult for workers across the economy, be they midwives or steelworkers, to express their democratic wishes and take industrial action in defence of jobs and services.

"The right to strike is a fundamental human right recognised across the democratic world. It has played a crucial role in helping workers - whether they are a member of a union or not - to secure better conditions and lives for them and their families.

"The Trades Union Bill is an opportunistic and ideological attack on unions at a time when the government should be focused on securing the UK's fragile economic recovery and creating better jobs to boost productivity."

A Business Department spokesperson said: "People have the right to know that the services on which they and their families rely will not be disrupted at short notice by strikes supported by a small proportion of union members.

"The right to strike is important but it is only fair that there should be a balance between the interests of union members and the needs of people who depend on their services."

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